Chicken immunoglobulins will be examined in order to obtain information on immunoglobulin C-region complexity, evolution, a- d biological functions. The proposal embodies the following questions: (1) What is the extent of C-region gene duplication in chicken immunoglobulin heavy and light chains? (a) Are chicken immunoglobulin light chains controlled by one or more genes? (b) Have recent gene duplications occurred in chicken heavy chain C-regions giving rise to subclasses? (2) Do structural homologies exist among chicken and mammalian immunoglobulins which may serve as indicators of evolutionary relatedness and/or related biological functions? (a) Is the chicken 7.8S heavy chain equivalent to the gamma chain of man? (b) Is the chicken secretory immunoglobulin equivalent to mammalian IgA? (3) By manipulation of the B cell population, is it possible to regulate the production of immunoglobulin classes previously undetected?